What's a carb? Besides being insanely delicious, we mean. Basically, carbs are a category of molecules in grains, produce, dairy and sweets that occur in three key forms.

Sugars

These are the easiest to break down into glucose, a major fuel source. They exist naturally in food (fructose in fruit; lactose in dairy); it’s added sugar you should worry about. Sweeteners such as table sugar, agave nectar and honey contribute calories without other good-for-you stuff, like fiber in fruit or protein in milk.

Fiber

This complex carb functions in almost the opposite way from sugars: You can’t digest it, and certain fibers move slowly through the digestive tract, helping you feel full, shuttling out some fat and slowing glucose absorption. Aim for 25 grams a day, says SELF contributing expert Janis Jibrin, R.D.

Starches

Also complex carbs, but not all starch is created equal. Take the starch in potatoes. It’s complex compared with sugars but still very easy to break down into glucose. Resistant starch—found in beans, lentils and whole grains such as barley and whole wheat, among others—is indigestible and functions like fiber. Processing is a factor, too. Refining whole grains strips their fibrous shell, so they act more like sugars in the body; cooking and cooling potatoes turns some of their starch resistant. Confusing? Yep. The boiled-down takeaway: Avoid refined carbs, seek whole grains, and don’t sweat the rest.

Should I ever carbo-load?

Consider it when you’re training competitively for a 90-plus-minute endurance event, says Roberta Anding, R.D., instructor at Baylor College of Medicine. “Carbs are exercising muscles’ preferred food,” she says. But nix the prerace pasta bonanza: You can’t store that much extra glucose. Instead, up your carbs five days or so prior, to about 8 g per 2.2 pounds of body weight a day, and trim fat to avoid weight gain.